Development of many of the world's petroleum reserves is hindered or prevented by the nature of crude oil where the viscosity, pour point and API gravity renders the crude oil unsuitable for pipeline transportation. Varied methods of producing pipeline-quality oil from such crudes have been used. In general, such methods can be categorized as either physical or chemical treatments.
Physical treatments change the physical properties of the oil to produce a pumpable fluid, but do not change the chemical composition of the oil itself. As discussed by Flournoy et al. in U.S. Pat. No. 4,134,415 (1979) a common method involves dilution of the heavy crude with lighter fractions of hydrocarbons. This can involve the use of large amounts of expensive solvents to transport a relatively cheap product and requires the availability of the diluent which can be inconvenient in certain oil fields. Another method disclosed by Flournoy et al. involves heating the heavy oil to reduce its viscosity. This method requires the installation of heating equipment along the pipeline and insulation of the pipeline itself. Such a procedure is expensive and uses a large amount of energy. The extent of decrease in viscosity which can be achieved by an increase in temperature varies widely between heavy oils depending on the oil composition. Such physical treatments do not upgrade, i.e. enhance the value of, the oil and, in fact, usually increase the overall cost of oil processing. Nevertheless, physical treatments provide a simple solution and are most widely used today. In many applications, dilution with lighter crudes is coupled with pipeline heating for pumping very heavy crudes. It is also possible to add water to reduce the pressure gradients as discussed by B. L. Moreau in an article "The Pipeline Transportation of Heavy Oils", The Journal of Canadian Petroleum Technology, p. 252, 1965. However it is difficult to maintain proper flow in this system and still obtain the desired viscosity reduction. Other methods such as the addition of surfactants to form oil-in-water emulsions have been used. Flournoy et al., U.S. Pat. No. 3,943,954 (1976).
Chemical treatments can involve contacting the oil with a strong base to form an oil-in-water emulsion which is more easily transported. Kessick et al., Canadian Pat. No. 1,137,005 (1982). However, chemical treatments typically require changing the hydrogen to carbon ratio of the oil, either by reducing the carbon content or by addition of hydrogen. Carbon reduction technologies range from simple distillation and deasphalting to mild visbreaking to severe thermal cracking. Distillation and deasphalting processes result in separation of the heavy portion of the oil, i.e. the residuum, from the remaining lighter portion, with only the lighter end being transported.
A number of processes which involve heating a heavy oil to improve its transportability have been tried over the years. A thermal treating process to reduce the viscosity and improve transportation of the oil has been disclosed by Engle in U.S. Pat. No. 3,496,097 (1970). This process involves heating the oil between 500.degree. F. and 700.degree. F. for at least 24 hours. The process has the disadvantage of being time and energy consumptive and producing substantial amounts of gas which are not readily used in the field.
Scott et al. in U.S. Pat. No. 3,474,596 (1969) describe a process for reducing the viscosity of a stream of viscous fluid flowing within a pipeline by diverting a portion of the stream and heating it to about 850.degree. F. to 900.degree. F. (454.degree. C.-482.degree. C.) and 200 to 400 psig at which thermal degradation or "visbreaking" of at least some of the constituents thereof takes place. This heated portion is then blended with the remainder of the stream to reduce the viscosity of the bulk material. This process, however, only modifies a portion of the oil. Additionally, that portion which is modified must be taken from the fraction of "dry oil" which is obtained from a crude oil-water separator.
Huang in U.S. Pat. No. 4,298,455 (1981) discloses that the pumpability of a heavy hydrocarbon oil, such as a crude, reduced crude or other oil with an API gravity of less than 15.degree., is improved by using a viscosity reducing or visbreaking heat treatment. The disclosed process involves heating the oil at between 800.degree. F. and 950.degree. F. (427.degree. C.-510.degree. C.) between two and thirty minutes and at a pressure of 100 to 1500 psig. To minimize the amount of coke or tar and gas formed during this visbreaking process, the visbreaking is carried out in the presence of a chain transfer agent and a free radical initiator. This process requires the careful control of the concentration of the initiator and transfer agent in conjunction with adjustment of the residence time at reaction temperature to minimize coke formation.
A method which involves reducing the viscosity and sulfur content of a heavy crude as it is being produced is disclosed by Meldau in U.S. Pat. No. 3,442,333 (1969). This method involves injecting steam at the wellhead through a conduit which extends down-hole. The steam heats the oil to a temperature in the range of 550.degree. F.-700.degree. F. (288.degree. C.-371.degree. C.). The rate of production of the oil is controlled so that the oil is at temperature within the well for at least 24 hours. This process has the disadvantages of long contact times at temperature, high energy requirement, low production rates, and the necessity for special equipment in each well-hole.
A form of thermal cracking known as visbreaking is well known in the art. As disclosed by Biceroglu et al. in U.S. Pat. No. 4,462,895 (1984), visbreaking conditions can include temperatures from 750.degree. F.-950.degree. F. (399.degree. C.-510.degree. C.) and pressures of 50-1500 psig. Other conditions disclosed include a temperature of 850.degree. F.-975.degree. F. (454.degree. C.-524.degree. C.) and a pressure of 50-600 psig. Beuther et al. U.S. Pat. No. 3,132,088 (1964). Normally the residue from "topped" or "reduced" crudes is the feedstock for refinery visbreaking operations. Taff et al. U.S. Pat. No. 2,695,264 (1954). It has been disclosed by Beuther et al. in U.S. Pat. No. 3,324,028 (1967) that resids and certain heavy crudes with an API gravity below about 20.degree. can be exposed to visbreaking conditions. This patent, however, teaches that the resids or crude should be hydrodesulfurized before visbreaking at 800.degree. F.-1000.degree. F. (427.degree. C.-538.degree. C.) at pressures of 50-1000 psig. Such "visbreaking" processes are not practical for in the field treatment of whole crude because of the additional facilities required to pretreat the feedstock and to recover and process products from the treatment.
The principal variables in single-pass visbreaking have been reported to be furnace outlet temperature, residence time and pressure. Beuther et al., "Thermal Visbreaking of Heavy Residues", The Oil and Gas Journal, Vol. 57, No. 46, p. 151 (1959). An increase in any of the three variables is said to result in an increase in visbreaking severity. Shu et al. in U.S. Pat. No. 4,504,377 (1985) and Yan et al. in U.S. Pat. No. 4,522,703 (1985).
It has been disclosed that at higher severities there is an increased tendency to form coke deposits in the heating zone or furnace. Black in U.S. Pat. No. 1,720,070 (1929) teaches that operating at lower temperatures for increased lengths of time provides "a much smaller amount of carbon is deposited than is deposited at higher temperatures." Hanna et al. in U.S. Pat. No. 1,449,227 (1923) disclose the continuous circulation of a stream of oil from an evaporating chamber through a heating coil to maintain the temperature of the oil in the chamber at the desired cracking temperature. The temperature differences between the oil in the chamber and the heating coil is kept small to minimize cracking in the coil. Hess in U.S. Pat. No. 1,610,523 (1926) teaches that it is desirable to avoid local overheating in order to prevent excessive coke formation in cracking systems of oil distillation. Akbar et al., "Visbreaking Uses Soaker Drum", Hydrocarbon Processing, May 1981, p. 81 discloses that, when there is a high temperature differential between the tube wall in a furnace cracker and the bulk temperature of the oil, the material in the boundary layer adjacent to the tube wall gets overcracked. Therefore, the coking rate is roughly a function of the inside boundary layer temperature. In furnace cracking this boundary layer is commonly 30.degree. C.-40.degree. C. higher than the bulk temperature. In soaker cracking the skin temperature is lower but still is reported to be above 480.degree. C. Therefore, the formation of coke is slower in a soaker cracker but still causes regular shutdowns of the equipment for coke removal.
Frequent shutdowns for coke removal from visbreaking units can be tolerated in refinery operations where there is adequate storage for the topped crude or residue feedstock normally processed. However, this is unacceptable in a field operation where crude is continually produced and must be rapidly transported. Yan et al. (supra) recognize the problem of coke formation. They attempt to minimize the problem by adding "1-10 weight percent of finely divided solids in the heavy hydrocarbon oil feedstream . . . " in an attempt ". . . to prevent the deposition of coke on the walls of the heating coils and reactor . . . "
Although some patents relating to visbreaking suggest that whole crude can be used as a feedstock, this has not proven possible with conventional processes due to the pressure generated by the volatile components present in the whole crude. In fact, Lutz in U.S. Pat. No. 4,454,023 (1984) teaches that it is necessary to pass a whole crude oil through a distillation column before passing it to a visbreaking heater. Black (supra) teaches that it is desirable to minimize vaporization during cracking to maintain only a liquid phase. Black used mechanical pressure of up to 1000 psi and the addition of a liquid diluent to maintain the liquid phase.
In view of the disadvantages of the processes described hereinabove, there is a need for a process suitable for well-site locations by which viscous crudes can be rendered more pumpable. More particularly, it would be advantageous to have a process which, unlike traditional visbreaking, is suitable for untopped, rather than topped, feeds and which uses lower temperatures to achieve the same or greater viscosity reductions.
It has now been found that significant reductions in the viscosity of heavy hydrocarbon mixtures can be attained with a process using a vertical tube reactor. Vertical tube reactors which oridinarily involve the use of a subterranean U-tube configuration for establishing a hydrostatic column of fluid sufficient to provide a selected pressure are known. This configuration provides a less expensive way to achieve high pressures than with standard high pressure pumps. This type of reactor has been primarily used for the direct wet oxidation of materials in a waste stream and particularly for the direct wet oxidation of sewage sludge.
Bower in U.S. Pat. No. 3,449,247 discloses a process in which combustible materials are disposed of by wet oxidation. A mixture of air, water and combustible material is directed into a shaft and air is injected into the mixture at the bottom of the hydrostatic column.
Lawless in U.S. Pat. No. 3,606,999 discloses a similar process in which a water solution or suspension of combustible solids is contacted with an oxygen-containing gas. Excess heat is removed from the apparatus by either diluting the feed with the product stream or withdrawing vapor, such as steam, from the system.
Land, et al. in U.S. Pat. No. 3,464,885 (1969) is directed to the use of a subterranean reactor for the digestion of wood chips. The method involves flowing the material through counter-current coaxial flow paths within a well-bore while flowing heated fluid coaxially of the material to be reacted. The reactants, such as sodium hydroxide and sodium sulfate, are combined with the wood chip stream prior to entry into the U-tube which is disposed within a well-bore.
Titmas in U.S. Pat. No. 3,853,759 (1974) discloses a process in which sewage is thermally treated by limiting combustion of the material by restricting the process to the oxygen which is present in the sewage, i.e. no additional oxygen is added. Therefore, it is necessary to provide a continuous supply of heat energy to effect the thermal reactions.
McGrew in U.S. Pat. No. 4,272,383 (1981) discloses the use of a vertical tube reactor to contact two reactants in a reaction zone. The method is primarily directed to the wet oxidation of sewage sludge in which substantially all of the organic material is oxidized. There is heat exchange between the inflowing and product streams. The temperature in the reaction zone is controlled by adding heat or cooling as necessary to maintain the selected temperature. It is disclosed that when gas is used in the reaction, it is preferred to use a series of enlarged bubbles known as "Taylor Bubbles". These bubbles are formed in the influent stream and are transported downward into the reaction zone. It is disclosed that preferably air is introduced into the influent stream at different points with the amount of air equaling one volume of air per volume of liquid at each injection point. The presence of this amount of oxidant would not be possible with a liquid which was primarily carbonaceous.
Other patents which disclose the use of a hydrostatic column to generate pressure include Beddoes, U.S. Pat. No. 887,506 (1908). Silverman in U.S. Pat. No. 3,371,713 (1968) discloses a method for generating steam for steam flooding for oil production. Palmer in U.S. Pat. No. 1,514,098 (1924) discloses a system in which an elevated vessel is used to provide a low pressure hydrostatic head on oil in a thermal cracking vessel. Other patents include U.S. Pat. No. 3,140,986 of Hubbard (1964) and U.S. Pat. No. 2,421,528 of Steffen (1947).
The above-cited patents which disclose vertical tube reactor systems describe the use of such systems with primarily aqueous streams. None of these patents describe treatment of a primarily hydrocarbon stream. Specifically, there is no suggestion of the thermal treatment of a hydrocarbon stream in a vertical tube reactor system to provide for viscosity reduction. Based on the teachings of the visbreaking art as described hereinabove, it would be expected that coking of the reactor surfaces would be a significant problem with this configuration.
Therefore, it would be advantageous to have a thermal process by which significant viscosity reduction can be achieved with a heavy oil feedstock. It would be particularly advantageous for the process to produce little or no coke make so that a vertical tube apparatus could be used. Additionally, the process should provide viscosity reduction without the need for long residence times and a high throughput rates.
These and other advantages are now achieved by practice of the present invention as described hereinbelow.